The steam generators of pressurized-water nuclear reactors comprise a bundle of tubes bent into a U shape and comprising two straight legs whose ends are fixed in a circular tube plate.
The two straight legs of each of the tubes are joined by a curved part whose radius of curvature depends on the position of the tube in the bundle. The bundle comprises tubes whose ends are fixed in openings passing through the tube plate in regions at various distances from its periphery. The tubes whose ends pass through the tube plate near its periphery comprise a curved part with a large radius of curvature, whereas the tubes passing through the tube plate near its central part comprise a curved part with a very small radius of curvature.
The various curved parts of the tubes of the bundle are placed in adjacent positions and form the upper part of the bundle.
The tube plate comprises an entry face with which the ends of the tubes of the bundle are level, this entry face of the tube plate forming the upper wall of a water box arranged in the lower part of the steam generator.
The water box is divided by a partition into two parts and each of the tubes of the bundle opens out into one of the parts of the water box via one of its ends and into the other part of the water box via its other end. The pressurized water of the nuclear reactor is injected into one of the parts of the water box which is responsible for distributing it into the tubes of the bundle; after circulating in the tubes, this pressurized water is then collected in the second part of the water box.
For steam generator repair or maintenance operations, it may be necessary to insert a tool or a measuring or monitoring device into a tube of the bundle or into a certain number of tubes in succession.
For example, in order to reduce the corrosion sensitivity of the curved parts of the tubes of the bundle, the stress in this curved part is relieved by inserting inside the tube an electrical heating element which is positioned in the curve and is then supplied with electrical current.
These operations are performed on the steam generators of nuclear reactors, which are contaminated after the reactor has been in operation for some time.
It is therefore necessary to perform the insertion and the positioning of the heating elements in the curves of the tubes of the bundle from a distance and from outside the water box of the steam generator.
This is done by employing a means for pulling and pushing, extended by a guide conduit which can be inserted into the water box and positioned in the extension of any tube of the bundle by a device such as a manipulator arm, arranged inside the water box. The means of pulling and pushing allows a flexible transmission member of elongate shape to be moved inside the guide conduit and into any tube of the bundle from outside the water box. The transmission member comprises an end fitted with a coupling which is inserted into, and then moved in, the tube by pushing until this coupling reemerges from the other end of the tube into which it was inserted.
An additional guiding means makes it possible to recover the end of the transmission member comprising the coupling at an opening of the water box.
An electrical heating element is attached to the coupling outside the water box and is then inserted into the tube by pulling on the transmission member which drags the electrical heating element by means of the coupling.
The pulling movement is performed over a sufficient length to position the electrical heating element along the entire length of the curve in which the stress relieving is performed.
The elongate transmission member which is moved by pushing and by pulling inside the tubes of the bundle, and which is generally referred to by those skilled in the art as a "rabbit", comprises a sheath whose external diameter is smaller than the internal diameter of a tube. Satisfactory guidance of the rabbit inside the tube is thus obtained.
However, a device of this kind does not allow the push to be transmitted in a wholly satisfactory manner when it is being positioned The rabbit tends to bend in successive sections and to jam inside the tube, this phenomenon being known as "snaking". The friction of the rabbit's sheath inside the tube increases until it is no longer possible to continue the movement by pushing. These problems increase with the length of the tubes along which the rabbit is to be moved.
It is possible to avoid these disadvantages to some extent by taking precautions when the rabbit is being moved and by performing this movement very slowly. As a result, the positioning of the rabbit is tricky and increases the time of exposure of the operators to the radiation originating from the steam generator.
Moreover, when the tool is positioned in the curve by pulling the rabbit, the latter tends to lengthen because of the considerable friction between the sheath and the inner surface of the steam generator tube. It is therefore very difficult to position the tool accurately in the curved part of the tube. This defective positioning entails a risk of compromising the satisfactory progress of the operation performed by the tool.
Friction between the sheath of the rabbit and the internal surface of the tube requires very large pulling forces, particularly in the case of the curved parts of small radius of curvature which are situated in the central part of the bundle In this case, it becomes difficult, or even impossible, to position correctly a tool such as a heating element inside the curve.